Sims will wear their assigned formal attire for a family gatherings.
Wedding Party Outfit Fix is a modular mod, but you can use each module on its own or combine them in whatever way you like.
Required Packs: My Wedding Stories
Compatiblity: Will conflict with any mod that alters these tuning files:
jobs_PrePostWeddingParties_Guest_FamilyGathering, 278284
jobs_PrePostWeddingParties_Player_FamilyGathering, 278285
Creator website: https://www.seebee.de/sims4me/